Power consumption is important in most electronic devices, and is especially important in battery-powered electronic devices. In order to reduce power consumption in these battery-powered devices, chips or circuits in these devices are often designed to maintain a low power standby mode of operation when not in use. Consequently, substantial operating power may be conserved. It is often difficult, however, to quickly bring chips or circuits out of standby mode to fully operational, active mode in response to the occurrence of certain events or conditions, such as interrupts or flags. To maintain quick operational responsiveness to such events or conditions, a wake-up timer is sometimes employed to bring the chip or circuit from the low-power, inactive state to a fully operational active state. Such a timer remains active while the rest of the circuit is in an inactive, standby mode. Accordingly, when the event or condition occurs, the wake-up timer must "wake", or bring out of a standby mode, the chip or circuit. For many applications, it is desirable for battery-powered electronic devices to be designed to operate in an inactive, standby mode as long as possible.
For one particular digital wireless telephone consisting of a battery-powered handset and a wall-powered base station, the handset and base station communicate with each other over a predetermined frequency band through the use of digital radio transmissions. When the base station receives an incoming call from the Central Office, or similar exchange such as a private PBX, it begins to continuously transmit digitally encoded messages to the handset. When the handset detects these ringing and link establishment messages coming from the base unit, it immediately responds with digital messages of its own to inform the base station that it is within range and is capable of receiving a call.
A typical battery-powered, wireless telephone handset operates in an inactive, low power standby mode where the handset is not listening for messages from the base unit. The handset also operates in an active mode where the handset scans for the presence of base station messages. When in its standby mode, the handset must periodically wake up from the low power mode to search for incoming ring messages from the base station. If the handset stays in this inactive mode too long, it will miss the initial active ringing period corresponding to the first valid ring signal received from the Central Office or similar central telephone system. One particular design requirement for cordless telephones is to provide an audible signal in response to the receipt of the first valid ring signal. This audible signal is needed as soon as possible to inform a user of the existence of the incoming call. If the handset is in its inactive mode for the entire 2 second time period of the initial ring signal, the handset would fail to provide an audible signal to the user for a period of delay of up to 6 seconds after the receipt of the first ring signal. This delay period of up to 6 seconds is unacceptably long. As such, cordless telephone handsets are typically configured and arranged to provide some type of audible signal in response to every ring signal asserted by an attached telephone system. Many handsets are configured to remain in the inactive mode for approximately one second before waking up to scan for incoming ring messages. If no ring messages are found, the handset re-enters the inactive state for a one second period of time when the process repeats. The one second time period is chosen to ensure that the handset scans for ring messages during a portion of every two second ring signal, and thus timely provide the audible signal.
The drawback to having the handset wake up frequently is that the handset uses a significant amount of power every time it wakes up to scan for ring messages which are usually not present. This significant amount of power consumption significantly limits the life of a charged battery in the handset. The amount of power taken from the battery is directly proportional to the amount of time the handset spends in the active mode. Therefore, the useful life of the battery could be extended if the amount of time the handset spends in the active mode is decreased.